The present invention relates to a laser system and more particularly to a miniature pulsed laser system using coupled resonator cavities and to a method of producing a beam of coherent light which is eyesafe.
Increase in the use of lasers in recent years has produced a requirement for lasers of higher power that are safe for the human eye. The greater the power of the laser, the more risk there is to people who may come into contact with the laser beam when a coherent beam of light enters the eye cornea and either passes through or is absorbed by the vitreous humor. The portion of the beam that is not absorbed by the vitreous humor is focused by the eye onto the retina. Under normal conditions, the light energy is converted by the retina into chemical energy to stimulate optical sensation. Injury can result to the eye when the focused energy laser beam cannot be absorbed and causes damage to the retina. This damage does not occur when conventional sources of illumination are exposed to the eye because the light is emitted in all directions and produces a sizeable but not a focused image on the retina that can be safely absorbed. Laser beams having wavelengths in the range of 1.5 .mu.m -2.2 .mu.m are absorbed by the vitreous humor, thereby alleviating damage to the retina. Laser systems used as optical radar and communication transmitters in populated locations need to be operated so as to avoid eye damage.
Lasers operating in the 1.5 .mu.m-2.2 .mu.m wavelength have generally been of low efficiency and of larger size. Two available eyesafe lasers are based on laser emissions in erbium-doped solid state host materials pumped by pulsed gas discharge lamps or frequency conversion of a neodymium laser using stimulated raman scatter in a molecular gas, such as methane. These devices, however, have shortcomings. The erbium lasers typically have an efficiency of less than 0.1% owing to the low stimulated emission coefficient of the laser transition in erbium 3+ ion at a 1.54 .mu.m output and to the low efficiency for optical pumping with a visible flashlamp. The erbium laser can only be operated in a pulsed mode. Stimulated Raman conversion requires a cell containing a high pressure flammable gas. This gas is excited by the neodymium pumped laser to emit stimulated radiation in the eyesafe region. Raman conversion therefore is not amenable to continuous wave operation and the Raman process deposits energy in the conversion medium causing thermal distortion so that the eyesafe Raman laser cannot be conveniently operated at high average power or repetition rate.
An article in Optics Communications, Volume 75, No. 3,4 of Mar. 1, 1990, entitled Generation of Tunable Mid-IR (1.8-2.4 um) Laser From Optical Parametric Oscillation in KTP by J. T. Lin and J. T. Montgomery, describes an optical laser system in which an Nd:YAG laser is used in an optical parametric oscillator setup where the pumping beam of YAG laser pumps an optical parametric oscillator to produce an output in an eyesafe wavelength. Similarly, in the Burnham et al. U.S. Pat. No. 5,181,211, for an Eye-Safe Laser System, an Nd:YAG or Nd:YLF solid state laser is used to produce a polarized output beam which is passed through a non-linear crystal in an optical parametric oscillator to convert the wavelength of the pump laser to a wavelength that is absorbed by the human eye.
An optical parametric oscillator or OPO places a non-linear crystal within a resonant optical cavity in which mirrors transmit the pump wavelength from a laser beam through a non-linear crystal, such as potassium titanyl phosphate or KTP. The non-linear crystal can be rotated to change the output wavelength. The existence of a resonant optical cavity makes the parametric oscillator superficially similar to lasers since they also generate a coherent beam. However, since there is no stimulated emission within the parametric oscillator cavity, it does not act as a laser simply because the parametric oscillator is in a resonant optical cavity. The oscillator can be brought within the laser cavity.
The use of a short pulse (&lt;10 ns) Nd:YAG laser to pump a non-critically phased matched KTP optical parametric oscillator in the eyesafe region results in unacceptably low conversion efficiencies, such as less than ten percent. This low efficiency apparently was due to the short pump pulses. When the OPO was placed intracavity to the pump laser, the conversion efficiency increased but the output consisted of multiple pulses rather than a clean single pulse required for many applications.
The present laser system in contrast to the prior art uses coupled laser cavities to maintain the high efficiency of an intracavity system while at the same time achieving a single pulsed output to thereby overcome the problems of an extra cavity optical parametric oscillator used in combination with an Nd:YAG laser and also overcomes the shortcomings of placing the OPO intracavity to the pump laser. The present laser system is very compact for placement in very small packages which compactness has been accomplished using a single common substrate mirror with four separately coated regions and a single corner cube to form two primary laser resonators. A second smaller corner cube is used to couple the resonators.
A typical optical parametric oscillator apparatus in which the OPO is external of the laser may be seen in U.S. Pat. No. 4,180,751 to Ammann which has a laser having a laser cavity mounted adjacent a second resonant cavity of an optical parametric oscillator with the laser being directed into the optical parametric oscillator. In the Geiger et al. U.S. Pat. No. 5,195,104, an internally stimulated optical parametric oscillator and laser places the optical parametric oscillator within the laser cavity to form a dual optical resonator containing a single optical parametric oscillator and laser crystal intracavity. A frequency modified laser which places a non-linear crystal within the laser cavity can also be seen in the Anthon et al. U.S. Pat. No. 4,884,277.